samer.forums :
Atoms have their full SOC as well , all ARM chips as well. in time the Desktop will follow. it will save space on the motherboard and reduce cost as well.
How much external IO (PCIe, SATA, USB, display, etc.) do Atoms and ARM chips have? Only a fraction of what desktop CPUs have and on mobile/embedded chips, there is extra pressure on integration to save board space. Integrating the memory controller was an absolute necessity because accessing memory over the chipset was a major performance bottleneck and running the FSB fast enough to keep up with memory speed is also horribly power-inefficient.
Integrating the chipset into the CPU for desktop PCs wouldn't save any board space since the boards are made to fit standard form factors. Adding ~80sqmm to the CPU die size would also reduce the wafer yield by ~40%, considerably increasing CPU manufacturing cost, an inefficient use of an expensive fabrication process. If you want to reduce board size, you have to give up standard form factors and space for PCIe slots and other IOs, at which point you are in SoC territory and may as well go with an Atom.
EPYC gets around the lack of chipset by re-purposing PCIe lanes in a similar way to how Intel's chipsets have HSIO lanes that can be configured as either PCIe, USB3 or SATA by the motherboard manufacturer. Every IO on EPYC comes out of its PCIe budget. I'm a little surprised AMD didn't choose to do the same with ThreadRipper, guess it didn't feel like 64 HSIO lanes was enough to make that compromise yet.